Osmolarity calculator

Osmolarity in mOsm/L is grams per litre divided by molecular weight, multiplied by the number of particles the compound dissociates into, multiplied by 1000. Solutions above roughly 900 mOsm/L are generally given centrally, because high osmolarity damages peripheral veins.

When to use the osmolarity calculator

Use when assessing whether a parenteral solution can be given peripherally. Solutions above roughly 900 mOsm/L are generally given centrally, because high osmolarity damages peripheral veins and causes phlebitis.

The osmolarity formula

Osmolarity in mOsm/L = (grams per litre ÷ molecular weight) × number of particles the compound dissociates into × 1000.

Worked example

Dextrose 5% is 50 g/L with a molecular weight of 180 and does not dissociate, so osmolarity = (50 ÷ 180) × 1 × 1000 = 278 mOsm/L, close to plasma.

The mistake that costs marks

The particle count is what catches people. Dextrose does not dissociate so i is 1, but sodium chloride separates into two ions so i is 2, which doubles its osmolarity for the same molar concentration. Forgetting that halves the answer for every electrolyte solution.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• Milliequivalent and millimole converter — mEq and mmol
• Isotonicity calculator (sodium chloride equivalent) — Isotonicity (E-value)
• IV drip rate calculator — IV drip rate

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Bactericidal versus bacteriostatic. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.